In today's integrated circuit technology, on chip amplifiers play a key role in managing small signal delivery and usage. As an example, wireless communication devices are a combination of several functions that used to be separate devices. Currently, a digital camera is commonly embedded in a cellular telephone. There may also be a personal music player imbedded in the cellular telephone that allows the user to listen to music while not engaged in a telephone conversation. Some devices add wireless internet access and a personal data assistant in the same package. These devices challenge the design of power structures that support them. Each function puts additional drain on the batteries and induces variation in the level of power that is delivered to each. On chip amplifiers that utilize the fluctuating power may be subject to dynamic changes in performance due to the fluctuations.
Continuing with the example, wireless functions in the cellular telephone detect and amplify minute signals that are susceptible to being missed if the regulation of the amplifier line is not maintained. Most signal loss is not recoverable and may be detected by the user as a drop-out of the information being transferred. In the case of a cellular telephone call, the signal drop-out may sound like a “bad connection”. It may actually be the result of poor line regulation in the amplifier due to the demands on the power system.
Small signal systems classically rely on amplifiers utilizing pairs of transistors connected in a differential configuration. Since one of the transistors in the differential pair may be connected to an output driver while the other is not, the differential pair may be susceptible to variations in supply voltage. This condition may be caused by the difference in voltage at the drains of the transistors that make-up the differential pair. As an example, the drain of one of the transistors in the differential pair may be equal to VDD-VGS while the other transistor in the differential pair that is coupled to an output transistor may be equal to VOUT+VGS. Since the voltage at the drain of the first transistor is dependent upon VDD while the voltage on the drain second transistor is not dependent on VDD, any variation in VDD will contribute directly to a power supply dependent offset in the amplifier.
Certain configurations of the differential pair may provide a reduced susceptibility to the power supply dependent offset. If the differential pair is configured as a current mirror in a cascode structure, it will display less power supply dependent offset. Unfortunately this configuration may not supply sufficient output control to eliminate the power supply dependent offset. If extremely tight line regulation is required by the application, then even a cascode configured current mirror may not supply sufficient isolation from the power supply dependent offset.
It is possible to implement an output stage that couples to the differential pair and allows the drains of both of the transistors to be referenced to VDD-VGS. Though this option is possible, it is not always practical because it introduces another gain stage and restricts the current in the output stage relative to that of the input differential pair.
Thus, a need still remains for an integrated circuit system for line regulation of an amplifier. In view of the number of mobile devices that must manage multiple functions with only a battery as their power source, it is increasingly critical that answers be found to these problems. In view of the ever-increasing commercial competitive pressures, along with growing consumer expectations and the diminishing opportunities for meaningful product differentiation in the marketplace, it is critical that answers be found for these problems. Additionally, the need to save costs, improve efficiencies and performance, and meet competitive pressures, adds an even greater urgency to the critical necessity for finding answers to these problems.
Solutions to these problems have been long sought but prior developments have not taught or suggested any solutions and, thus, solutions to these problems have long eluded those skilled in the art.